Cannot run nested tests

## Environment

dotnet: 5.0.301
vscode: 1.58.0
extension: 0.7.5

## Background

Many of my tests have one or more inner classes, for organization, and for user-friendly output in cli and vscode tooling:
````cs
public class UserTests {

public class Inner1 {

public class Foo {

[Fact]
public void test1() { /* ... */ }

[Fact]
public void test2() { /* ... */ }

}

public class Bar {

[Fact]
public void test1() { /* ... */ }

[Fact]
public void test2() { /* ... */ }

}

}

}
````

That is shown in the test explorer like this:
````
UserTests
Inner1
Foo
test1()
test2()
Bar
test1()
test2()
````

## Issue

In the test explorer, there is a small "run" button next to every level and test.

The run buttons work for levels 1 and 2, but not for levels 3+. Those inner tests can only be run by running the parent, which runs many more tests than necessary, so it's slow.

Start by locating the test explorer's run-button handling and the code that builds commands for nested test nodes. Reproduce the failure with the shown nested classes; done means run buttons at level 3 and deeper execute only their selected tests.

Written by the indexing model from the issue text.